The invention relates to a variable-volume airbag for an airbag module.
In order to obtain a certain direction of extension in the initial phase of the unfolding of the airbag or when staged generators are used, it is expedient not to unfold the entire volume of the airbag immediately. When the last-mentioned generators are used, in the event of only a substage being activated, the airbag is filled more slowly and with less gas mass. The inadequate filling of the airbag delays the buildup of membrane forces in the airbag fabric and therefore the restraining force on the occupant. The forward shifting of the occupant would be very great and it would scarcely be possible to avoid contact with the rigid interior contour.
It is therefore know to unfold the airbag in this case with a reduced volume.
U.S. Pat. No. 5,362,101 thus discloses reducing the volume of the airbag by it being partially laid in folds which are sewn together. In the first phase after ignition of the gas generator, for example after ignition of the first stage of a two-stage gas generator, the airbag is unfolded only up to the reduced volume limited by the seams. When the internal pressure is increased by ignition of the second stage, the abovementioned seams are finally torn open and the airbag can be unfolded to its full volume.
The disadvantage of reducing the volume by means of tearing seams resides in the fact that the said seams have to be torn open automatically, but because of tolerances in the fabric and also because of environmental influences (for example, temperature, air humidity), a defined and reproducible tearing open of the seams can scarcely be achieved. Furthermore, the folding of an airbag having taken-in regions and the installation in the airbag module are also problematical because the airbag is distorted because of the darts and, as a result, handling in the production process is made more difficult.
Furthermore, airbags for airbag modules are known in which catch bands are arranged in order to obtain certain directions of unfolding and in order to limit the volume in the first unfolding phase, i.e. in an intermediate stage of the unfolding. The catch bands are provided both within the airbag and outside it. They are fastened by both ends to opposite sides of the airbag and are dimensioned in such a manner that they tear off from the airbag or even burst at a predetermined internal pressure. An airbag of this type with an external catch band is disclosed, for example, in the document JP-A-07 069149.
These catch band arrangements have the same disadvantages as in the abovementioned arrangements with tearing seams.
The invention is based on the object of achieving, with a relatively small outlay, a limitation of the volume of the airbag, the intention being for the increase in size of the airbag to its maximum volume to be possible with improved reproducibility.
In the case of a variable-volume airbag for an airbag module, in order to limit the airbag volume in an intermediate stage, at least one pull element is provided which is loosely guided on the airbag and after the airbag is gathered together is secured by its ends and which can be released or torn after being subjected to a certain stress during the unfolding of the airbag. A pull cable or a rigid element can be provided as the pull element. A cord can preferably be used as the pull cable. However, the term pull cable also includes a pull band.
In the present case, the pull cable is intended to gather together the airbag in such a manner that during the unfolding in a first phase a smaller volume has to be filled with gas than in a later phase. This is of importance, for example when staged generators are used. In these staged generators, in the event of only a substage being activated, the airbag is filled more slowly and with less gas mass. In the case of an airbag having a single, large volume, this results in it being inadequately filled, which delays the buildup of membrane forces in the airbag fabric and therefore the restraining force on the occupant. The forward shifting of the occupant is very great and it is not possible to avoid contact of the occupant with the rigid interior contour.
By adapting the airbag volume or the airbag shape by means of the pull cable to the gas mass provided in each case, and therefore to the filling pressure, this disadvantageous effect of the staged generator can be compensated for.
The reduction of the airbag volume by means of a pull cable or a plurality of pull cables also has the advantage that the said cables do not have to be connected to the airbag, as is required, for example in the case of catch bands. Rather, the ends of the pull cable can also be fastened at any desired location.
After the airbag is gathered together by means of a pull cable, its ends can be secured by being connected to one another. The pull cable then surrounds the gathered-together region of the airbag without being connected thereto. The reproducibility of the increase in size of the airbag volume is only influenced by the pull cable itself, but not by the airbag fabric and seams provided thereon. The airbag volume can be increased in size at a predetermined stress by the separation of the ends of the pull cable or by the tearing of the pull cable.
A further possibility of securing the ends of the pull cable resides in fastening at least one end in the housing of the airbag module. This results in further possibilities of releasing the pull cable by separately arranged mechanisms being provided for the release of at least one end. As a result, the reproducibility of the increase in size of the airbag volume can be increased further, since now even the strength of the pull cable which slightly fluctuates from specimen to specimen has no influence.
For the release at least of one end of the pull cable, for example, thermal or mechanical mechanisms can be provided or a detonating cap can also be provided.
The airbag can be gathered together by means of the pull cable in such a manner that it is partially constricted or is subdivided into chambers. In the last mentioned case, the pull cable lies over an angular range of at least 360xc2x0 around the airbag.
The pull cable is generally guided such that it bears closely against the airbag in guide elements. For example, tabs, rings or flexible tubes can be used as these guide elements.
The pull cable can be provided on the outside and/or inside of the airbag.
On the one hand, the pull cable can preferably be provided in regions of the airbag with which the occupant does not have contact after the unfolding of the airbag. This definitely rules out the possibility of the occupant being injured by an external pull cable after the latter has been torn off. On the other hand, a pull cable or a plurality of pull cables can be provided in different regions of the airbag in order to influence both the size and the shape of the airbag in the first phase of the unfolding.
Provision is made in one embodiment for a pull cable to extend over a predetermined length of the airbag at a constant distance from the edge of the blow-in mouth of the airbag, and for the two ends to be fastened in the housing of the airbag module. However, it is also possible for the guide elements to be provided alternating at a different distance from the blow-in mouth of the airbag, so that the pull cable runs in a zigzag shape.
In a further embodiment, starting from a first circumferential pull cable, at least one second pull cable is provided which runs in the direction of the blow-in mouth essentially perpendicularly with respect to the first pull cable.
In another embodiment, at least one eyelet is provided on the airbag for the guiding of the pull element.
In this embodiment, at least one pull cable can be provided which is connected fixedly to the airbag housing by one end, runs through the eyelet and is fastened by the other end in a retaining device which opens at a predetermined pressure.
In another refinement, at least one pull element is provided which is connected fixedly to the airbag housing by one end and whose other end is provided with a hook which engages in the eyelet and bends up at a certain tensile stress.
The invention will be explained in exemplary embodiments with reference to drawings, in which: